This invention relates to a process for the purification of fluorinated alkanoic acids such as, for example, perfluorooctanoic acid, from aqueous solutions of their salts.
The ammonium or alkali metal salts of fluorinated alkanoic acids are generally used in the aqueous emulsion polymerization of fluorinated olefin monomers such as tetrafluoroethylene, chlorotrifluoroethylene or vinylidene fluoride because of their high surface activity and low rate of telogen formation. In the course of these polymerizations, some of the fluorinated olefins hydrolyze to produce fluoride ions. After polymerization is complete and the resulting fluoropolymer is isolated, the fluorinated alkanoic acid ammonium or alkali metal salt is present in a dilute aqueous solution, along with ionic fluorides and other impurities. It is highly desirable to recover these fluorinated alkanoic acids and their salts for reuse, both because of their high cost and the need to minimize pollution problems. For such reuse, any impurities in the recovered compounds that may inhibit or change the course of the subsequent polymerization must be removed. Of particular use in such applications are the ammonium or alkali metal salts of fluorinated alkanoic acids, such as, for example, perfluorooctanoic acid (PFOA).
A known method of purification is by acidifying a fluorinated alkanoic ammonium or alkali metal salt solution by adding sulfuric acid until a pH of 1 to 2 is obtained, and subjecting the acidified solution to steam distillation. This is an energy-inefficient process, and yields a dilute solution of about 10-16 wt % fluorinated alkanoic acid. This must be neutralized with ammonia or alkali metal and concentrated for reuse by a means such as reverse osmosis. In an additional difficulty, the fluorides in the solution can co-distill during the steam distillation and corrode the distillation equipment.
U.S. Pat. No. 4,609,497 discloses a process for the recovery of ammonium or alkali metal salt of PFOA from a solution containing a selected nonionic polyether by extracting the ammonium salt with an organic liquid, decanting the organic layer containing the PFOA, contacting it with alumina to adsorb the PFOA, contacting the alumina with ammonium hydroxide to remove the PFOA , acidifying and decanting the resulting solution, steam distilling the organic phase in the presence of an acid and an oxidizing agent, and neutralizing the PFOA with ammonium hydroxide. This process is cumbersome to carry out, and requires the addition and removal of an organic extractant and an alumina adsorbent.
U.S. Pat. No. 4,282,162 discloses a process wherein, after a fluorinated carboxylic acid has been recovered from an aqueous solution by adsorption on a basic ion exchanger; it is eluted using a mixture of a mineral acid and an organic solvent. The eluate is then decanted, the lower organic layer neutralized and acidified, and the precipitated fluorinated carboxylic acid filtered. The fluorinated carboxylic acid obtained by this process is not of polymerization grade. U.S. Pat. No. 5,312,935 discloses a process for purifying such acid by oxidizing it in a solution containing less than 9% by weight of water. This process requires special ion exchange equipment, frequent replacement of the ion exchange resin, the addition and removal of an organic solvent, and a further purification step.
U.S. Pat. Nos. 5,442,097 and 5,591,877 disclose a process for recovery of fluorinated carboxylic emulsifiers from an aqueous solution containing its salt by acidification of the salt, reacting the acid with an alcohol to form the ester, distilling the resulting ester mixture, and decanting the ester layer. The resulting ester may be reacted with aqueous ammonia to form the corresponding ammonium salt. This process requires introduction of an organic reactant, which must be recovered, and requires carrying out additional esterification and de-esterification reactions.
The above processes are complicated and expensive to carry out, requiring multiple conversion steps in which the fluorinated alkanoic compound is either transferred to other media, such as an organic solvent or adsorbing solid, or converted to an organic ester, followed by transfer or conversion back to the fluorinated alkanoic compound. There is a need to develop a simpler process in which the fluorinated alkanoic acid can be isolated by an efficient means, and without such transfers or conversions. An advantage of the invention is that the invention process does not require transferring the fluorinated alkanoic acid to an organic solvent or an adsorbent, or its conversion to an organic ester. Another advantage of the invention is it does not require concentration of the purified ammonium or alkali metal salt solution with reverse osmosis, and it minimizes the content of corrosion-causing fluorides.
A process that can be used for isolating a fluorinated alkanoic acid from an aqueous solution also containing inorganic fluorides is provided. This process comprises (A) acidifying an aqueous solution comprising an ammonium or alkali metal salt of a fluorinated alkanoic acid to produce an acidified solution, (B) heating the acidified solution to produce an organic layer and an aqueous layer in which the organic layer comprises the fluorinated alkanoic acid, (C) separating and recovering the organic layer from the aqueous layer, (D) optionally washing the organic layer with an acid solution, (E) optionally isolating the fluorinated alkanoic acid, and (F) further optionally converting the fluorinated alkanoic acid to its ammonium or alkali metal salt.
The present invention can be used to isolate any fluorinated alkanoic acid. The fluorinated alkanoic acid can have the general formula of X-Rf xe2x80x94COOH in which X is hydrogen, fluorine, chlorine, or combinations thereof and Rf is a fluorinated group that can be saturated or unsaturated, linear or branched such as methyl-branched alkylene group having 5 to 12, preferably 5 to 10 carbon atoms per group. Generally the fluorinated alkanoic acid can contain some inorganic fluorides, from about 5 to about 1200, more typically about 10 to about 1000 mg/kg of the fluorinated alkanoic acid (ppm). The fluorinated alkanoic acid can be present as an aqueous solution of a metal or, preferably, ammonium salt.
Examples of such fluorinated alkanoic acids include, but are not limited to, perchlorofluoroalkanoic acids and perfluoroalkanoic acids. One such perfluoroalkanoic acid is perfluorooctanoic acid, also called perfluorocaprylic acid.
Optionally, the aqueous solution can be, before or after step (A), treated with an amount of a soluble aluminum salt. Preferably the amount of aluminum salt is at least equivalent to the inorganic fluoride content present in the aqueous solution. The treatment can be carried out for an effective period of time such that the inorganic fluorides are reacted to form mixed aluminum fluorides. The mixed aluminum fluorides can then be substantially removed in the separation step (C). Preferably this treatment is done before step (A). Also preferably the aluminum salt is soluble over the pH range of the process, and contains the same counter-ion as the acid to be used in step (A). For example, if the acid used in step (A) is sulfuric acid, the preferred aluminum salt is aluminum sulfate. The temperature of treatment can be any convenient temperature. This optional step is particularly preferred if the initial aqueous solution is high in fluorides. Using this procedure, the fluoride content of the fluorinated alkanoic acid and the amount of distillation residue can be reduced to an acceptable level, which means that the residue can be easily removed, for the recovery process.
Any acid that does not interfere or react with the fluorinated alkanoic acid can be used in step (A). The preferred acid is a mineral acid to react with the fluoroalkanoate and form the free fluorinated alkanoic acid, which is only slightly soluble in the mineral acid under these conditions. Examples of preferred acids are those well-known mineral acids. Also preferred is that the mineral acid contains less than about 50 volume % water because above 50 volume % can increase the amount of water present in the resulting mixture and therefore the solubility losses for the fluorinated alkanoic acid. If the final purification is distillation, the preferred acid is a relatively nonvolatile acid that does not co-distill during distillation, such as sulfuric acid, phosphoric acid, or nitric acid. The preferred acid is sulfuric acid because of its low volatility. If the final purification step is by crystallization or other means, any mineral acid is satisfactory.
The amount of acid is preferably an amount sufficient or effective to neutralize the ammonium or metal content of the fluorinated alkanoic acid salt and to provide an acid environment sufficient to minimize the solubility of the resulting fluorinated alkanoic acid. Generally the amount can be determined by the pH of the acidified aqueous solution, which can be any pH that can facilitate the isolation or separation of the fluorinated alkanoic acid. The pH can be as low as about 2 or lower. Excess acid, however, can be difficult to remove completely in later steps and is preferably avoided. For example, with sulfuric acid, the preferred amount is about 1 to about 4, preferably about 1 to about 3, and most preferably about 1.3 times the theoretical amount required for neutralizing the ammonium or metal ions, based on only one proton of the sulfuric acid being used.
In step (B), the acidified solution can be heated to a temperature and for a period of time sufficient or effective to produce an organic (lower) layer and an aqueous (upper) layer. Generally the temperature is preferably above the melting point of the wet fluorinated alkanoic acid. The wet melting point is appreciably lower than that of the dry or anhydrous fluorinated alkanoic acid because of the solubilizing effect of the contained water. For example, it was found the water content for perfluorooctanoic acid to be about 12-15% at this point in the process, and that it is a liquid at temperatures above 35-40xc2x0 C. in the process even though pure perfluorooctanoic acid has a melting point of 55-56xc2x0 C. The separation temperature may be between about 35 and 60xc2x0 C. A separation temperature of 45 to 50xc2x0 C. is preferred. At temperatures above about 60xc2x0 C., the solubility losses of perfluorooctanoic acid in the aqueous layer may become excessive. For other fluorinated alkanoic acids, the separation temperature is preferably only slightly above the melting point for the wet fluorinated alkanoic acid, and below the point where excessive solubility losses occur.
Wishing not to be bound by theory, if the heating is carried out at temperatures below the melting point of the wet fluorinated alkanoic acid, the fluorinated alkanoic acid may be initially present as small particles dispersed throughout the total liquid, making isolation difficult. However, upon raising the temperature above the fluorinated alkanoic acid""s melting point, these particles soon coalesce into a separate liquid phase. This phase can then be easily separated from the aqueous phase by simple decantation, since the phases form a relatively clean separation with a minimum interfacial layer.
Thereafter, in step (C) the aqueous upper layer is separated from the organic lower layer. Any means known to one skilled in the art can be used in the separation. Presently decantation is preferred for it is simple and easy to use. Separation removes most of the water and with it a major portion of the water-soluble impurities.
The recovered organic layer can be washed with a diluted acid solution. The acid can be the same as that used in step (A). The amount of the diluted acid is preferably effective to remove most of the dissolved acid. It is preferred that the diluted acid contains less than about 10 volume % acid to produce a purified fluorinated alkanoic acid. For example, about 5 volume % sulfuric acid can be satisfactory for the diluted or weak acid wash. One or more acid washes can be carried out if desired to minimize distillation residue. Excessive washing such as, for example, more than 3 washes, is preferably avoided because of solubility losses for the fluorinated alkanoic acid. The wash can be conducted at any temperature, preferably at about 10 to about 60xc2x0 C. more preferably about 35 to about 60xc2x0 C., and, most preferably 35 to 50xc2x0 C. A water wash is not recommended since it may create emulsion problems due to ionization of the fluorinated alkanoic acid.
The purified fluorinated alkanoic acid can be recovered from the washed organic layer by any means known to one skilled in the art such as, for example, distillation and crystallization. Distillation is generally preferred. For example, in a batch distillation, the first material overhead is generally the fraction that co-distills with the small amount of dissolved water by a steam distillation effect. This fraction can also contain most of any fluorides that remain, in the form of the volatile hydrogen fluoride. This fraction can be combined with the main product fractions or recycled back to the start of the purification process, depending on its amount and quality. After the dissolved water is removed, the temperature of the distillation generally rises to that of the boiling point of the fluorinated alkanoic acid, which can be then readily vacuum distilled to a receiving vessel. For fluorinated alkanoic acids, which are waxy solids at ambient temperature, such as perfluorooctanoic acid, it is preferred to keep tempered water on the condenser to keep them from freezing out during distillation and plugging the condenser. If desired, the molten product can be mixed with water to depress its freezing point, reacted with ammonia to a pH about 5.5, and the solids adjusted to a standard level for reuse as an emulsifier. The amount of distillation residue remaining is relatively small, depending on the efficiency of washing. It can be removed by refluxing with a sodium hydroxide solution.
The resulting purified fluorinated alkanoic acid from step (E) can readily be converted to the ammonium or alkali metal salt and reused in a polymerization reaction. For convenience, it can be neutralized to a pH of about 5.5 and standardized with water to an appropriate concentration of ammonium or alkali metal fluorinated alkanoate for subsequent use.
The process of the invention can be used for substantially increasing the concentration of a fluorinated alkanoic acid alkali metal or ammonium salt from a dilute aqueous solution of the fluorinated alkanoic acid ammonium or alkali metal salt as low as about 10 weight %. At the same time, the fluorinated alkanoic acid ammonium salt can be separated from nearly all impurities such as low molecular weight organic fluorides and inorganic fluorides, making it suitable for reuse in polymerization reactions.
The process can be run as a batch or continuous operation. Steps prior to distillation could be carried out in a single contacting device.
The efficacy of this simple process is surprising, since previously referenced U.S. Pat. Nos. 5,442,097 and 5,591,877 both disclose that, if the original aqueous solution of the salt is acidified with sulfuric acid, and attempts are made to distill the resulting fluorinated carboxylic acids, they fail since these acids form crystallizing hydrates which can block up the distillation equipment. Moreover, distillation residues are produced in this case which still contain considerable amounts of fluorine, the disposal of which is very complex. The inventive process avoids these difficulties.
Preferably the fluorinated alkanoic acid recovered in the invention process is a perfluorinated alkanoic acid, and still more preferably is perfluorooctanoic acid.
The following examples are intended to illustrate the inventive process, but should not be interpreted as limiting the scope of the invention in anyway.